docs: def com_google_fonts_check_metadata_unique_weight_style_pairs(family_metadata):
    """METADATA.pb: check if fonts field
       only contains unique style:weight pairs.
    """
    pairs = {}
    for f in family_metadata.fonts:
      styleweight = f"{f.style}:{f.weight}"
      pairs[styleweight] = 1
    if len(set(pairs.keys())) != len(family_metadata.fonts):
      yield FAIL, ("Found duplicated style:weight pair"
                   " in METADATA.pb fonts field.")
    else:
      yield PASS, ("METADATA.pb \"fonts\" field only has"
                   " unique style:weight pairs.")